Epoxy resin compositions are commonly used for the preparation of insulation systems for electrical engineering. However, most of these epoxy resin compositions utilize anhydrides as curing agents. Due to the developing regulatory framework for chemicals, it is expected that the use of anhydrides in epoxy resins will be restricted in the near future, because of their R42 label (respiratory sensitizer). Therefore, some anhydrides are already on the SVHC candidate list (substances of very high concern) of the REACH regulation. Therefore, it is likely that in some years these substances may no longer be used without special authorisation. As methyl hexahydrophthalic anhydride (MHHPA) and hexahydrophthalic anhydride (HHPA) are widely used as the main curing agents for cycloaliphatic outdoor epoxy resins for electrical insulation applications, there is a future need for alternative solutions that are not regarded as SVHC. As all known anhydrides are R42-labeled and even yet unknown anhydrides would be expected by toxicologists to be also R42-labeled, a solution that is free of anhydrides is desirable.
Amines as curing agents for epoxy resins are well known, in particular, for the preparation of composite materials. However, amine curing agents are often too reactive to be processable in electrical potting or encapsulation applications. As the mass of the epoxy resin composition to be processed increases, control of the exotherm becomes vital. The uncontrolled release of heat from the curing of the thermoset due to its mass may result in the degradation of the thermoset's mechanical properties, or even to thermal decomposition of the thermoset. Also degradation of the mechanical properties of the structural parts in contact with the thermoset is likely to occur. In particular in automatic pressure gelation process (APG), it is important to provide for a lower exothermic peak temperature to control the cure profile, i.e. gelation front within the mold. The cure profile of epoxy resin compositions is inappropriate and the exotherm is too high for application in APG, when amines are used as curing agents.
In order to cope with the problem of an inappropriate cure profile of epoxy resins containing amine curing agents, the use of aromatic amines was suggested. However, today the aromatic amines considered are on the banned substance list which prevents their use in potting or casting applications. As indicated above, other amines, such as aliphatic amines, are too reactive and do not provide an acceptable gelation profile in APG, which is suitable for the casting of big parts with low shrinkage and low exotherm. Moreover, some properties of the cured products are not competitive with anhydride cured thermosets, such as long term aging, tracking resistance, arc resistance, dielectric properties after humid conditioning. Accordingly, there is a need for new thermosetting, anhydride-free epoxy compositions which advantageously can be used in potting or encapsulation applications for manufacturing of electrical insulation systems, such as switchgear or transformer applications, suitable for outdoor applications.
It is an object of the present invention to provide a multiple component, anhydride-free thermosetting epoxy resin composition which is suitable for the preparation of articles exposed to outdoor conditions, such as outdoor insulation systems for electrical engineering. The epoxy resin composition shall be R42-free and SVHC-free, and distinguished by a low water pick-up, a very good water diffusion break down strength and a long pot life. The epoxy resin composition shall be suitable for processing by automatic pressure gelation (APG). It is desirable that the cure profile can be controlled in the desired manner. Still another object of the present invention is to provide the encased articles obtained from potting or encapsulation process which exhibit good mechanical, electrical and dielectrical properties, and can be used in outdoor applications, for example, as insulators, bushings, switchgears and instrument transformers in electrical engineering.